The present invention generally relates to gas generating systems and, more particularly, to On Board Inert Gas Generating Systems (OBIGGS).
Aircraft have used OBIGGS to protect against fuel tank explosions by replacing the potentially explosive fuel vapor/air mixture above the fuel in the ullage space of the tanks with nitrogen enriched air (NEA). The OBIGGS passes air (e.g. bleed air) through an air separation module (ASM), generating the NEA and a stream of oxygen enriched air (OEA). The resulting NEA can be used to inert fuel tanks while the OEA can be vented overboard. Various methods for improving the efficiency of OBIGGS have been described.
In U.S. Pat. No. 6,491,739 an OBIGGS having a fast warm up feature is disclosed. The described system uses a valve, referred to as a Fast Start Valve, to assist in warming up the ASM during start up conditions. The '739 patent also uses a modified ASM. The ASM modification involves adding an inlet port to the OEA side of the ASM. This modification enables use of the Fast Start Valve. The Fast Start Valve introduces either warm bleed air or NEA into the OEA inlet port thus warming up the ASM faster. The OEA is vented overboard through the “normal” OEA vent port. Although the described system may reduce the delay in reaching optimum operating temperatures for permeable membrane (PM) based systems, greater NEA generating efficiency and improved OBIGGS are still needed.
Another OBIGGS is disclosed in U.S. Pat. No. 7,048,231. The described system uses a turbine driven compressor to increase engine bleed air pressure to generate the NEA. The turbine expands the NEA, which produces the power to drive the compressor. The OEA is vented to the ram air circuit through a check valve. The check valve is used to prevent back-flow of air from the ram air circuit. For applications requiring greater NEA production, the '231 patent boosts the system flow by using a “co-axial boost motor” and/or bleed air from the high-pressure segment of the aircraft's Air Cycle Machine (ACM). Although the increased inlet air pressure may improve NEA production, operating a boost motor and/or tapping into the aircraft's ACM reduce aircraft efficiency.
Another OBIGGS has been disclosed in U.S. Patent Application No. 2006/0021652. The described OBIGGS uses a modulating valve at the NEA outlet. The modulating valve is used to vary the NEA flow so as to minimize the changes to the oxygen content within the fuel tank. As the modulating valve closes, the NEA oxygen concentration decreases and OEA flow increases and vice versa as the valve opens. The OEA is vented overboard through the ram air circuit. The modulating valve in the NEA stream is used to control NEA production. The amount of NEA required is based upon a desired flow, which is calculated based on a number of factors, including fuel tank pressure, temperature, and oxygen concentration. The objective is to minimize either the fuel tank oxygen concentration or to minimize the changes to oxygen concentration. The described system also includes an ejector and an ejector shutoff valve in the OEA duct. The ejector and shutoff valve admits high-pressure air bleed air into the OEA duct. This effectively reduces the pressure at the OEA port and increases OEA flow and decreases the NEA flow and NEA oxygen concentration thereby enhancing NEA purity, (lower NEA oxygen concentration). The shut-off valve is used to turn off the ejector when NEA enhancement is not required. Use of the ejector can only decrease the NEA flow and reduce NEA oxygen concentration. Additionally, the ejector's use of bleed air reduces aircraft efficiency. The described system does not have the ability to directly control NEA oxygen concentration or to increase NEA flow.
As can be seen, there is a need for improved gas generating systems. In some situations, the production of NEA is not sufficient to produce an inert atmosphere. Under these situations, one would normally increase the inlet airflow, inlet pressure, or inlet temperature, or a combination of the three. However, when these options are not available, a need arises to improve the nitrogen generating efficiency of the ASM.